4D Reservoir Monitoring with Gravity Measurements and the Potential Application to Carbon Dioxide Sequestration

Precision measurements of Earths gravitational field with modern gravity meters can give useful information about subsurface reservoir fluid movement. A change in pore fluid density gives a change in the bulk rock density which is potentially detectable at the surface. Trial surveys over two depleted gas reservoirs now used as gas storage reservoirs have proved that reservoir fluid movement can be detected with gravity monitoring. Both continuous and time-lapse readings were taken. At the Hatfield Moors reservoir a high level of noise due to changes in the shallow water table was detected. However, at the Izaute reservoir a good correlation with the gas stock and pressure data was achieved. When the spatial pattern of maximum gravity change over the reservoir is compared with the forward modelled response, which assumes homogenous reservoir properties, a good first order correlation is observed. However, there are areas of the maximum gravity change pattern that do not fit with the modelled response. These anomalous areas could be showing areas of the reservoir that have different porosity and permeability. This technique has many applications in reservoir monitoring, particularly where 4D seismic is unsuccessful or expensive, and also as a monitoring tool for CO2 sequestration.<br>